The present invention relates to a method intended to provide mixing of at least one gaseous fluid such as air and of a fuel in the combustion chamber of a direct-injection internal-combustion engine, notably of Diesel type.
The invention also relates to an internal-combustion engine using such a method.
It is already well-known in conventional-combustion Diesel engines to use a particular intake for the gaseous fluid, such as air or a mixture of recirculated exhaust gas and air, so as to favour mixing of this fluid and of the fuel injected.
Some of the known methods for favouring this mixing consist in transmitting a swirling motion to the gaseous fluid, i.e. a rotating motion around an axis that is substantially parallel to or merges with the axis of the combustion chamber, either after its intake in the combustion chamber or as it enters the combustion chamber so that the gaseous fluid mixes by  less than  less than swirling greater than  greater than  with the fuel injected in form of fine droplets.
This swirling motion of the gaseous fluid can be created by at least one intake manifold arranged tangentially and radially to the combustion chamber, and referred to as tangential manifold. The gaseous fluid thus flows in along the wall of the cylinder and it generates a swirling motion around the principal axis of the combustion chamber. It can also be created by at least one manifold of helical shape, referred to as helical manifold, which is designed in such a way that the gaseous fluid already has a swirling motion as it enters this combustion chamber. It is also possible to associate at least one tangential manifold with at least one helical manifold to generate this swirling motion.
This swirling motion of the gaseous fluid is known to the man skilled in the art as swirl and it is characterized by a ratio equal to ND/N, where ND is evaluated by integration on the path of the piston, during the intake stroke, of the rotation of the elementary feed introduced by taking account of the valve lift and of the piston speed, followed by division by the total amount of air (or of gaseous fluid) introduced, N being the engine speed.
This swirl affords the advantage of improving mixing of the gaseous fluid with the fuel while decreasing emissions such as fumes. It is more particularly of interest under low-speed and low-load running conditions of the engine when the internal aerodynamics of the combustion chamber is insufficient to provide mixing of the gaseous fluid with the fuel.
However, if too high a swirl ratio is used, it has the non insignificant drawback of diverting circumferentially the fuel jets in vapour phase from the injection nozzle and of leading to a configuration where the fuel jets are superposed on one another, which is harmful as regards fumes discharge.
Thus, in conventional-combustion Diesel engines, there is a compromise, for each working point, between the maximum swirl ratio allowing to limit emissions at partial load and a swirl ratio compatible with the injection characteristics, such as the nappe angle of the fuel jets, the diameter of the injection nozzle ports, the number of ports of this nozzle, the fuel injection pressure, so as to prevent the fuel jets from superposing on one another at high loads. Generally, a rather high swirl above 3 is always selected.
In the case of Diesel engines working under homogeneous combustion conditions, it is a more or less homogeneous mixture of gaseous fluid and of fuel that self-ignites, and it is therefore important to favour mixing of the gaseous fluid and of the fuel.
In order to favour this homogeneous mixing, the fuel and the gaseous fluid are brought together at an early stage in the cycle. A high swirl ratio would theoretically be beneficial because of the swirling induced between the gaseous fluid and the fuel, but the fuel droplets may be thrown onto the walls of the cylinder and thus dilute in the oil.
This fuel ejection would not only lead to a degradation of the behaviour of the lubricant present on this wall and to the creation of soots with a risk of piston sticking in the cylinder, but also to an emissions increase and to a decrease in the engine performances.
The present invention is aimed to overcome the aforementioned drawbacks by means of a method and of an engine allowing to obtain better mixing of the gaseous fluid with the fuel injected in the combustion chamber while allowing homogeneous combustion at low load and conventional combustion at high load.
The invention thus relates to a method providing mixing of at least one gaseous fluid in the combustion chamber of a direct-injection internal-combustion engine comprising at least a cylinder, a cylinder head, a piston sliding in this cylinder, a fuel-injection nozzle, a combustion chamber delimited on one side by the upper face of the piston comprising a teat pointing towards the cylinder head and arranged in a concave bowl, and intake means for at least one gaseous fluid, said intake means being designed so as to admit the gaseous fluid into the combustion chamber with a swirling motion, characterized in that the fuel is injected with an injection nozzle having a nappe angle less than or equal to       2    ⁢    Arctg    ⁢          CD              2        ⁢        F              ,
where CD is the diameter of the cylinder and F the distance between the point of origin of the fuel jets from the injection nozzle and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC), and the gaseous fluid is injected with a swirl ratio less than or equal to 1.7.
The gaseous fluid can be injected with a swirling motion coaxial to that of the bowl.
The fuel can be injected with a nappe angle less than or equal to 120xc2x0.
The fuel can be injected with a nappe angle ranging between 40xc2x0 and 100xc2x0.
It is possible to use a piston with a teat such that the angle at the vertex of said teat is substantially in accordance with the nappe angle of the injection nozzle and a bowl whose wall is shaped in such a way that the injected fuel is guided towards the outside thereof and vaporized without reaching the walls of said cylinder, for any position of the piston up to xc2x130xc2x0 in relation to the top dead center (TDC).
The invention also relates to an internal-combustion engine comprising at least a cylinder, a cylinder head, a piston sliding in this cylinder, a fuel-injection nozzle, a combustion chamber delimited on one side by the upper face of the piston comprising a teat pointing towards the cylinder head and arranged in a concave bowl, intake means for at least one gaseous fluid, said intake means being designed so as to admit the gaseous fluid into the combustion chamber with a swirling motion, characterized in that this engine comprises at least one injection nozzle for injecting fuel with a nappe angle less than or equal to       2    ⁢    Arctg    ⁢          CD              2        ⁢        F              ,
where CD is the diameter of the cylinder and F the distance between the point of origin of the fuel jets from the injection nozzle and the position of the piston corresponding to a crankshaft angle of 50xc2x0 in relation to the top dead center (TDC), and the intake means are designed to admit the gaseous fluid into the combustion chamber with a swirl ratio less than or equal to 1.7.
The intake means can comprise at least one intake manifold designed to admit the gaseous fluid with a swirl ratio less than or equal to 1.7.
The intake means can comprise throttle means and the engine can comprise at least one control means for actuating the throttle means so as to obtain a swirl ratio less than or equal to 1.7.
The nappe angle of the injection nozzle can be selected between 0xc2x0 and 120xc2x0.
The nappe angle of the injection nozzle can be selected between 40xc2x0 and 100xc2x0.
The angle at the vertex of the teat is selected greater than the nappe angle by a value ranging between 0xc2x0 and 30xc2x0.
The bowl can have an inclined lateral wall and the angle of inclination of the wall is less than 45xc2x0.